A. Field of the Invention
The present invention relates to optical communication networking devices. More particularly, the invention relates to an optical fiber protection switch to protect optical networks, and specifically, wavelength division multiplexed optical 4-fiber Bi-Directional Line Switched Ring (BLSR) Networks.
B. Description of the Related Art
1. Optical Networking
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized optical digital transmission systems that are used, respectively, in North America and internationally. SONET networks typically use synchronous add/drop multiplexers (ADM) to add and/or drop asynchronous DS-n signals onto the links. The ADM devices also re-route signals to avoid faulty communication links. This is referred to as span and ring switching.
2. Span Switching and Ring Switching
Span switching and ring switching are mechanisms to re-route traffic over optical networks. A ring is a network configuration that allows signal path redundancy between nodes on a network by interconnecting the nodes in a loop, or ring. In a four-fiber ring, the nodes are connected with a pair of working fibers creating a bi-directional communication path, and a pair of protection fibers creating a second bi-directional communication path to be used as protection for the working ring. The working and protection fibers connect each node to the two adjacent nodes in the ring topology.
Span switching is performed between two nodes to re-route working traffic over the protection fiber in the event of a fiber failure on the working ring. The failure may be due to a fiber cut or signal degradation due to other equipment failures. The working traffic is placed on the protection fiber by the transmitting stations, then re-routed to the working fiber/ring at the receiver, thereby bypassing the failed fiber/equipment. The working traffic from the failed fiber span is thus re-routed to the protection fiber span.
Bi-directional line switched ring (BLSR) is a bi-directional ring that protects against fiber failures that are more severe—such as fiber bundle cuts—where the failure occurs in both working fibers and both protection fibers between two nodes on the ring. In the event of a fiber bundle cut between two adjacent nodes, the working ring traffic is re-routed in the reverse direction along the protection fiber ring by the nodes on either side of the fiber bundle cut. Automatic protection switching (APS) may be used to perform the ring switch automatically upon signal loss.
Current SONET BLSR uses a SONET add-drop multiplexer (ADM) as the line termination equipment, and APS performs the span or ring switch when transmission failure occurs within the ring. In the SONET BLSR architecture, the SONET ADM performs span and ring switching by bridging the working line with the protection line, thereby protecting the working traffic.
3. Wavelength Division Multiplexing
The demand for bandwidth in the transport network has been increasing at an exponential rate. This aggressive demand has fueled the rapid deployment of wavelength division multiplexing (WDM) in the network to alleviate fiber constraints. WDM may be viewed as a parallel set of optical channels, each using a slightly different light wavelength, but all sharing a single transmission medium. WDM systems have the advantage of increasing the capacity of existing networks without the need for expensive re-cabling.
In the current deployment scenario, a single, multi-wavelength WDM system transports multiple, concentric, single-wavelength SONET rings. SONET ring protection is typically accomplished by ADMs, providing both span and ring protection switching. Capacity increases on the WDM ring are typically accomplished by building a new SONET ring, which includes SONET ADMs to support the protection requirements.